1. Field
This invention relates in general to semiconductor memories, and, in particular, to the design and operation of multilevel nonvolatile semiconductor memories.
2. Description of Related Information
As the information technology progresses, the demand for high density giga bit and tera bit memory integrated circuits is insatiable in emerging applications such as data storage for photo quality digital film in multi-mega pixel digital camera, CD quality audio storage in audio silicon recorder, portable data storage for instrumentation and portable personal computers, and voice, data, and video storage for wireless and wired phones and other personal communicating assistants.
The nonvolatile memory technology such as ROM (Read Only Memory), EEPROM (Electrical Erasable Programmable Read Only Memory), or FLASH is often a technology of choice for these application due to its nonvolatile nature, meaning it still retains the data even if the power supplied to it is removed. This is in contrast with the volatile memory technology, such as DRAM (Dynamic Random Access Memory), which loses data if the power supplied to it is removed. This nonvolatile feature is very useful in saving the power from portable supplies, such as batteries. Until battery technology advances drastically to ensure typical electronic systems to function for a typical operating lifetime, e.g., 10 years, the nonvolatile technology will fill the needs for most portable applications.
The FLASH technology, due to its smallest cell size, is the highest density nonvolatile memory system currently available. The advance of the memory density is made possible by rapidly advancing the process technology into the realm of nano meter scale and possibly into the atomic scale and electron scale into the next century. At the present sub-micro meter scale, the other method that makes the super high-density memory system possible is through the exploitation of the analog nature of a storage element.
The analog nature of a flash or nonvolatile storage element provides, by theory, an enormous capability to store information. For example, if one electron could represent one bit of information then, for one typical conventional digital memory cell, the amount of information is equal to the number of electrons stored, or approximately a few hundred thousands. Advances in device physics exploring the quantum mechanical nature of the electronic structure will multiply the analog information manifested in the quantum information of a single electron even further.
The storage information in a storage element is hereby defined as a discrete number of storage levels for binary digital signal processing with the number of storage levels equal to 2N with N equal to the number of digital binary bits. The optimum practical number of discrete levels stored in a nonvolatile storage element depends on the innovative circuit design method and apparatus, the intrinsic and extrinsic behavior of the storage element, all within constraints of a definite performance target, such as product speed and operating lifetime, with a certain cost penalty.
At the current state of the art, all the multilevel systems are only suitable for medium density, i.e. less than a few tens of mega bits, and only suitable for a small number of storage levels per cell, i.e., less than four levels or two digital bits.
As can be seen, memories having high storage capacity and fast operating speed are highly desirable.